Animation involves the production of consecutive images, which when displayed, convey a perception of motion. The most common form of two-dimensional animation is sprite animation. A sprite is a bitmap image or a set of images that are composited over a background, producing the illusion of motion. Sprite animation is relatively fast and easy with modern computers. Also, sprites are typically selected from a library of suitable image. Consequently, if in an animation sequence the sprite can be identified, the sprite can be searched for within the library and the results used in further inferring the animation sequence, its context and other details.
Sprites for encoding video data is the focus of U.S. Pat. No. 5,943,445 issued Aug. 24, 1999 and assigned to Digital Equipment Corporation and entitled “Dynamic Sprites for Encoding Video Data”. U.S. Pat. No. 5,943,445 describes segmenting frames into rigid and non-rigid bodies, and identifying these bodies as sprites. Some suitable techniques for such encoding are presented in U.S. Pat. No. 4,853,775 issued Aug. 1, 1989 and assigned to Thomson-CSF and entitled “Method and Device to Estimate Motion in a Sequence of Moving Pictures”. Both these United States patents relate to estimating local motion vectors by performing gradient operations.
Sprite-based encoding is described in the U.S. Pat. No. 6,205,260 issued Mar. 20, 2001 and assigned to Sharp Laboratories of America, Inc and entitled “Sprite-based Video Coding System with Automatic Segmentation Integrated into Coding and Sprite-building Process”. This reference describes analysing MPEG video and disassociating the foreground from the background, with no restrictions on the moving objects etc. Complex mathematical transformations, such as affine transformations, perspective transformations, warping operations etc, are extensively used to separate a sprite from its background.
The above-described two references describe techniques that are relatively complicated, and involve complex mathematical operations.
In view of the above observations, a need clearly exists for techniques that are able to identify sprites against a background.